1. Field of the Invention
The application relates to a method utilized in a wireless communication system, and more particularly, to a method of reference cell maintenance in a wireless communication system.
2. Description of the Prior Art
Toward advanced high-speed wireless communication system, such as transmitting data in a higher peak data rate, LTE-Advanced system is standardized by the 3rd Generation Partnership Project (3GPP) as an enhancement of Long-Term Evolution (LTE) system. LTE-Advanced system targets faster switching between power states, improves performance at the cell edge, and includes subjects, such as bandwidth extension, coordinated multipoint transmission/reception (COMP), uplink multiple input multiple output (MIMO), etc.
For bandwidth extension, carrier aggregation is introduced to the LTE-Advanced system for extension to wider bandwidth, where two or more component carriers are aggregated, for supporting wider transmission bandwidths (for example up to 100 MHz) and for spectrum aggregation. According to carrier aggregation capability, multiple component carriers are aggregated into overall wider bandwidth, where a user equipment (UE) can establish multiple links corresponding to the multiple component carriers for simultaneously receiving and transmitting. In carrier aggregation, the UE only has one radio resource control (RRC) connection with the network. At RRC connection establishment/re-establishment/handover, one serving cell provides the Non-Access Stratum (NAS) mobility information, and at RRC connection re-establishment/handover, one serving cell provides the security input. This cell is referred to as a primary cell (PCell). In the downlink, the component carrier corresponding to the PCell is the Downlink Primary Component Carrier (DL PCC) while in the uplink it is the Uplink Primary Component Carrier (UL PCC). In addition, cells other than the PCell are named secondary cell (SCell).
Since the UE may not need to use all of the configured cells (i.e. PCell and one or more SCells), only some SCells are activated, so as to save UE power. Note that, the PCell is always activated. Generally, the eNB activates or deactivates a SCell by sending a signalling (e.g. a medium access control control element (MAC CE), or Activation/Deactivation command) to the UE. In addition, the UE starts a deactivation timer for a SCell when the SCell is activated, wherein the SCell is deactivated when the deactivation timer expires. In other words, the deactivation timer provides a period of time for SCell activation. Please note that, when the SCell is deactivated, the UE does not need to monitor physical downlink control channel (PDCCH) of the SCell. Further, the UE shall not transmit sounding reference signal (SRS) on the SCell, on uplink shared channel (UL-SCH) of the SCell, and not report channel quality indicator (CQI), precoding matrix indicator (PMI), rank indicator (RI), and precoding type indicator (PTI) for SCell.
As abovementioned, it is possible to configure a UE of a PCell and one SCell or more SCells. Therefore, multiple timing alignments, each for synchronization with a serving base station on uplink timing for preventing signals transmitted from the UE from colliding with those sent from other UEs under the coverage of the base station, are needed for PCell and SCell or more SCells. Note that, serving cells having uplink to which the same timing alignment applies are grouped in a timing alignment (TA) group. Each timing alignment group contains at least one serving cell with configured UL, and the mapping of each serving cell to a timing alignment group is configured by the serving eNB. In order to realize uplink timing alignment, the UE maintain a time alignment timer (TAT) whose running state indicates that uplink transmission is still synchronized. TAT may be applied for a TA group. Functionality of the TAT shall be well-known in the art, so it is not given herein.
Note that, in carrier aggregation, a UE may apply information (i.e. parameter settings) to one cell by referring to the information observed from other cell, which is called reference cell in this disclosure. For example, the reference cell may be a timing reference cell or a pathloss reference cell. For the timing reference cell, if a UE uses the downlink timing of Cell #x as the timing reference for the Cell #y, then call Cell #x is the timing reference cell of the Cell #y. As to the pathloss reference cell, a UE may determine uplink transmission power based on a lot of parameters. One of parameters may be the pathloss estimation. If UE determines uplink transmission power (e.g. PUSCH or PUCCH transmission power) for the Cell #y based on the downlink pathloss estimation from the Cell #x, then the Cell #x is the pathloss reference cell of the Cell #y.
A SCell could be in activation or deactivation state. In addition, a UE may lose its reference cell because reference Cell could be de-configured, de-activated, or suffer poor channel condition (e.g. radio link problem or radio link failure (RLF)). However, reference from a deactivated (or de-configured, or poor channel condition) SCell (i.e. a timing reference cell or a pathloss reference cell) may be less accurate, for example, due to less frequent measurements on the deactivated SCell.